Speech transmission system



Dern s, 1942,

K M. R. KLEls'r Y SPEECH TRANSMISSION -SYSTEM Filed Feb.. l'28, 1942 '3ShetS-Sheet l /NVENIUR n MR/(LE/ST ATTORMS'V Dem 8, 1942. M. R. KLEls'TSPEECH TRANSMISSION SYSTEM Filed Fem 2s, 1942 hs sheets-sheet 2 hwg.

ATTORNEY 5 Sheets-Sheet 3 De. 8, 1942. M. RKLEIST` SPEECH TRANSMISSIONSYSTEM Filed Feb. 2a, 19742 N hub HSM L Nm.. R.

Patented Dec. 5i, i942 SPEECH TRANSMISSION SYSTEM Myron l. Kleist, WestEnglewood, N. J., assigner to Bell Telephone Laboratories, Incorporated,New York, N. Y., a corporation of New York This invention relates to aspeech transmission system of the type which includes telephonerepeaters spaced at intervals along the transmission line andparticularly to such a system utilizing repeaters of the type whichincludes repeating coil hybrids.

An object of the invention is to enhance the adaptability of suchsystems to various operating situations.

Another object of the invention is to electrically simulate, byinterconnecting windings of two repeating coils, a single repeating coilhaving an impedance ratio substantially equal to but slightly greaterthan that of the two coils.

A more specific object of the invention is to permit one or more of thecircuits of a phantom group to be dropped or carried through on anonrepeatered basis while the remaining circuit, or circuits, of thegroup is repeatered, this being accomplished without appreciablereduction in the' transmission efciency of the overall system.

In Patent 2,282,464 issued May 12, 1942, to Messrs. P. G. Edwards and E.R. Taylor, entitled Speech transmission system, there is disclosed(Figs. 4 and 5) a speech transmission system utilizing repeating coilhybrids; it is in connection with a system of this general nature thatthe present invention is particularly applicable.

As disclosed in the patent referred to, the repeating coil hybridsordinarily comprise two coils with separate cores. Each coil is providedwith two line windings, the two upper line windings of the coils beingconnected in series with each other and with one side of the line andthe two lower line windings being connected in series :"5

with each other and with the other side of the line; the terminals ofthe respective upper and lower line winding which are not connecteddirectly to the line are connected as dictated by the particular system.Two balancing network windings are provided on each core; these windingsare connected to a balancing network common .to the two coils. Inaddition, one coil of each hybrid is provided with an out hybrid pair ofwindings and the other coil of the hybrid is provided with an in hybridpair of windings. In the following description this arrangement will begenerally referred to, when comparing it to the modiiication invented byapplicant, as a normal repeating coil hybrid.

In the use of a system of the general type referred to it often is founddesirable that one or more of the circuits of a phantom group be droppedat an intermediate point, or carried through on a non-repeatered basis,whilethe remaining circuit, or circuits,'of the group is re' peatered inthe normal manner. It is obvious, however, that it is at the same timedesirable that the overall system retain its previous electricalsymmetry so that phantom balance will be maintained and for otherreasons that will be discussed subsequently. It is particularlydesirable, for eX- ample, that the drop side of each of the modifiedrepeating coil combinations be reasonably well balanced to ground sothat the normal method of applying and controlling composite signalingcurrents may be followed at the non-repeatered point. Further, thecondition requiring the above referred to modication of the circuitsoften is of a temporary nature and it may become desirable after arelatively short interval to restore all circuits of a phantom group toarepeatered basis. It is desirable therefore that the arrangement be suchthat a change from one condition Ato the other can be accomplishedreadily.

A feature of the present invention is an arrangement of repeating coilwindings whereby changing a circuit from a non-repeatered basis to arepeatered basis, or vice versa, necessitates only a simple change ofthe conductive strapping of the windings.

A further feature of the invention is an arrangement of repeater coilwindings whereby a reasonably good balance to ground is maintained onthe drop side of each combination of repeating coils.

A still further feature of the invention is an arrangement of repeatingcoil windings whereby, in the instance of a repeater point at whichcertain circuits are on a non-repeatered basis, direct lmanner effectiveto appreciably reduce transmission loss.

In accordance with a specific embodiment of the invention, the basicarrangement of the repeating coils is the same regardless of the numberof circuits, if any, of a phantom group that are nonrepeatered. In theevent that a circuit is on a non-repeatered basis, however, certain ofthe windings of the repeating coils associated therewith are connectedin a novel parallel-aiding manner whereby a good balance to ground ismaintained on the drop side of the coils and other desirable operatingcharacteristics are assured A complete understanding of the variousnovel arrangements contemplated by the present invention as well asappreciation of the various desirable features thereof will be gained byconsideration of the following detailed description in connection withthe annexed drawings in .whichz Fig. 1 is a schematic showing of atransformer circuit provided to facilitate subsequent explanation ofpractical applications of the invention;

Figs. 2 and 3 illustrate schematically a speech transmission systemutilizing repeating coil hybrids, certain of the circuits being droppedat the intermediate repeater point and certain of the repeating coilwindings being modified in accordance with features of the presentinvention; and

Fig. 4 illustrates schematically the terminal section of one sidecircuit including switching means for automatically modifying thearrangement of certain of the repeating coil hybrid windings uponrepeater failure.

Referring now to Fig. 1, this figure is intended to represent thegeneral arrangement of any two transformers provided with primarywindings connected in series aiding and secondary windings connected inparallel aiding. (This, in general, is the arrangement of the modifiedrepeating coil arrangement utilized in non-repeatered circuits inaccordance with the novel features of applicants invention).

Theimpedance ratio of the coil of Fig. 1 may be written as:

(Jani/Lm. 2 van or as the equivalent:

i By reversing the poling of any one of the windings we obtain theimpedance ratio -Obviously if winding turns N are used rather thaninductance L the above equations can be written:

If the transformers of Fig. 1 be of the Ytype provided with multisectionwindings (this type of transformer is ordinarily used in the type ofsystem to be described subsequently) numerous impedance ratios can beobtained by utilizing the various sections in different combinations.This may be desirable in a long speech transmission system wheredifferent impedance conditions are often encountered at differentswitchboards.

Referring now to Figs. 2 and 3 which should be joined left to rightrespectively, there is illustrated schematically a portion of a speechtransmission system comprising a west terminal repeater section, anintermediate repeater section and an east terminal repeater section. Thesystem illustrated is substantially the same as that illustrated byFigs. 4 and 5 of the patent referred to above except for the fact thatresistance hybrids are utilized at the ofiice side of the terminalrepeaters instead of repeating coil hybrids and the fact that the voicefrequency circuits of side circuit No. 2 are dropped at the intermediaterepeater point (i. e. are on a non-repeatered basis), the arrangement ofthe repeating coil windings at that point being modified in accordancewith features of the present invention.

At the west terminal repeater section; input path I I of west-eastamplier I2 and output path I3 of east-west amplifier I4 are connected inconjugate relationship with each other and in energy transferringrelationship with line section I5 of side circuit No. 1 by theresistance hybrid and associated balancing network I5. Output path II ofamplifier I2 and input path 2| of amplifier I4 are connected inconjugate relationship with each other and in energy transferringrelationship with lines 22 and 23 by repeating coil hybrid 24 andassociated balancing network 25.

Voice frequency currents, impressed on line section I5 by the voicefrequency equipment of the west terminal of side circuit No. 1, dividein the branches of the resistance hybrid one-half passing into outputpath I3 of east-west amplifier I4 and being lost and the useful halfpassing into input path II of west-east amplifier I2. These currents,after amplification by west-east amplifier l2, are impressed throughwindings of repeating coil hybrid 24 on lines 22 and 23. As

output path I'I and input path 2I are coupledl in conjugaterelationship, no part of these amplified currents enters input path 2|of east-west amplifier I4.

The voice frequency currents pass over lines 22 and 23 to theintermediate repeater section where they again divide in the windings ofrepeating coil hybrid 26, one-half passing into output path 21 ofeast-west amplifier 4I and being lost and the useful half passing intoinput path 42 of west-east amplifier 43. These currents, afteramplification by west-east amplifier 43, are

impressed through windings of repeating coil hybrid 44 on lines 45 and4E for transmission to the east terminal repeater section. As the outputVoutput path 5I of east-west amplier 52 and being lost and the usefulhalf passing into input path 53 of west-east amplifier 54. Theseamplified currents are impressedon line section 55 through theresistance hybrid and pass thereover to the east terminal voicefrequency receiving apparatus of side circuit No. .1.. Since the outputpath of amplifier 54 and the input path of amplifier 52 are connected inconiugate relationship by the resistance hybrid and associated balancingnetwork 56. no part of the amplified currents enters the input path ofeast-west amplifler 52.

It will be understood that transmission of voice frequency currents overside circuit No. 1 from east to west .is accomplished in a mannersimilar v 1'2 ina manner generally similar `to that described above.

It will be observed, further, that a phantom circuit 'is derived at thewest terminal for transmission of voice frequency currents produced bythe voice frequency transmitting equipment associated with line section13. This derivation is over lines 14 and l5; line 14 is connected tolines 22 and 23 of side circuit No. l between capacitances 'i6 and 'VIwhile line 'l5 is connected to lines 'H Vand 12 of side circuit No. 2between capacitances 8| and 82.

The voice frequency currents originating in line section 13 areamplified by west-east amplifier 83 before they are impressed on thephantom circuit referred to above. This amplifier and the otherequipment units of the .phantom terminal are similar to thecorresponding units of the west terminal equipment of said circuit No. 1and operate in a generally similar manner.

Considering now the intermediate repeater section, it will be observedthat the intermediate section equipment of side circuit No. 2 differsfrom that of side circuit No. l, described above, in that the voicefrequency circuits of side circuit No. 2 are terminated at theintermediate section. This means of course that the two repeatered pathsthat we find in the equipment of side circuit No. l at thel intermediaterepeater section are not provided for side circuit No 2. f

The arrangement of the repeating coil hybrids is modified in accordancewith features of the present invention.

Considering first repeating coils 84 and 85, it will heV observed thatthe general arrangement, i. e. number and location of windings, etc., isthe same as that of the normal repeating coil hybrids for example hybrid2B. (For convenience of illustration windings 8S of repeating coil 84,which correspond to the out hybrid windings of repeating coil hybrid 28,have been shown on the upper end of the core rather than on the lowerend.) However the windings of repeating coils 84 and 85 are sointerconnected that they act as a single repeating coil' of properimpedance and not as a combined repeating coil and hybrid coil as do thenormal repeating coil hybrids such as hybrid 26.

It will be observed that the arrangement of line windings |82, |03 and|84 of repeating coils 84 and 85 is the same as that of thecorresponding line windings of repeating coil hybrid 26; these linewindings are connected in series aiding relationship with respect toeach other.

As the coils function as a single repeating coil il:

and not as a combined repeating coil and hybrid, no balancing networkneed be provided; windings |85, which correspond to one of the pairs ofbalancing network windings of a normal repeating coil hybrid are leftopen while windings iS, corresponding to the other pair of balancingnetwork windings ofl a normal hybrid, are connected in series withwindings ll of the same coil which correspond to the in hybrid windingsof normal repeating coil hybrid v:26. Windings 8D in turn are connectedin parallel aiding relationship with respect to windings |86 and l'l;joining the two cores by this path in effect electrically merges coils84 and 35 and causes the combination to function as a single coil. Itbecomes apparent therefore that we have in the arrangement of coils 84and 85 the equivalent of that illustrated in Fig. l and described above,i. e. a repeating coil having the primary windings (line Windings in theinstance of coils through, repeatered, basis.)

greater so that the transmission elciency of ther overall system notonly is not affected unfavorably by the necessary lack of symmetry, atthe intermediate repeater point but actually isdefinitely improved forthe non-repeatered circuits of side circuit 2. It will be readilyapparent that the arrangement is a particularly valuable one from aneconomic standpoint as, should it become desirable to establishtheintermediate circuits of side circuit No. 2 on a through, repeatered,basis, it would be a simple.y matter to rearrange coils 84 and 85 tocomprise a normal repeating coil hybrid such as hybrid 26. (As pointedout above the reason for establishing certain of the circuits on anon-repeatered basis is often of a temporary nature and it may becomedesirable after a short time to restore the circuits to a Otheradvantages of the arrangement particularly its adaptability to phantomderivation (no special facilities need be provided for maintaining`phantom balance) and to the transmission of direct current and lowfrequency alternating current supervisory signals, will be apparent fromsubsequent portions of the description. Another particularly valuablefeature resides in the fact that both secondaries are balanced to groundto facilitate composite signaling.

It is believed that the operation of coils 84 and 85 in reception andtransmission of voice frequency currents will be readily apparent. Itwill be recalled that it was pointed out above that voice frequencycurrents originating in the voice frequency equipment at the westterminal of side circuit No. 2 are applied to lines 1| and 12. Thesevoice frequency currents pass over lines 'H and l2 Vto the intermediatesection where they traverse line windings ||l|, |82, |03 and |04 of`repeating coils 84 and 85. These currents are impressed, in turn, on theT and R leads of the local switchboard circuit through inductive actionbetween the primary (line) windings` and secondary windings |66, |87 and86 of coils 84 and 85.

As pointed out above a feature of the novel arrangement disclosedresides in the fact that the drop side of `the coil is well balancedr toground; this balance results from the particular manner in which thesecondary windings are arranged. This is a particularly desirablefeature as it permits the connection across lines ||0 and i il ofcomposite signaling equipment of the type requiring the two lines to bewell balanced to ground if the equipment is to operate satisfactorily;such equipment is ordinarily used for mid-point or locall signaling andmay comprise,

for example, relay l I5 which is provided with two parallel operatingwindings, one winding being in series with line ll) and ground and the`other winding being in series with line and battery.

There is, of course, no through transmission between the west and eastterminals over side circuit No. 2 for voice frequency currents but. f

such currents originating in the local circuits associated with the Tand R leads of repeating coil 85 may control the direct currentimpressed upon the lines 1| and 12 by the composite signaling or dialingequipment. This direct current is transmitted from the intermediatesection over side circuit No. 2 to the west terminal ina manner similarto that described above. Similarly, voice frequency currents and directcurrent may be transmitted in both directions between the east terminaland the local switchboard circuits associated with the windings ofrepeating coils H3 and ||4; the arrangement of coils ||3 and ||4 issimilar to that of coils 84 and 85.

It will be recalled that it was pointed out above that a phantom circuitfor transmission of Voice frequency currents originating in line section13 of the west phantom terminal is derived from side circuits Nos. l and2; this phantom circuit is derived, as was pointed out, over lines 14and 15.

The phantom circuit voice frequency currents upon reaching theintermediate repeater section are picked off by lines |3| and |32, line|3| being connected to lines 22 and 23 of side circuit No. 1 betweencapacitances |33 and |34 and line |32 being connected to lines 1| and 12of side circuit No. 2 between capacitances |35 and |36. The phantomvoice frequency currents are amplified at the intermediate point bywest-east ampliiier |31 of the phantom circuit intermediate repeatersection after which they are again impressed on side circuits Nos. 1 and2 over lines |4| and |42, respectively. i

It will be observed that despite the fact that the intermediate sectionsof the two side circuits are physically unsymmetrical due to the factthat circuits of side circuit No. 2 are dropped,`the novelinterconnection of the repeating coils of side circuit No. 2contemplated bythe present invention results in an arrangement wherebythe normal derivation of the phantom circuit is not disturbed andwhereby the phantom circuit voice frequency currents may be picked offat the intermediate repeater point, amplified and returned to the sidecircuits with no additional problem whatsoever arising from the factthat circuits of side circuit No. 2 are non-repeatered, This is, ofcourse, a particularly valuable feature not only in view of resultingadvantages when the circuits are as disclosed but also in view of thefact that should the intermediate section circuits of side circuit No. 2subsequently be placed on a repeatered basis, such change can beintroduced without necessitating any change in the phantom connections.

Upon reaching the east terminal the phantom circuit voice frequencycurrents are again picked off the side circuits by lines |43 and |44and, after amplification by west-east amplifier |45, are applied by theresistance hybrid to line section |46 and transmitted thereover to thevoice frequency equipment of the east phantom terminal.

It will be understood that transmission of voice frequency currents overthe phantom circuit from east to west is accomplished in a mannersimilar to that just described.

Composite signaling currents for example dialing signals, may be appliedto lines 22 and 23 by composite signaling equipment |6| throughcomposite set retardation coils |62 and |64 which are connected,respectively, in series with the upper pair of line windings ofrepeating coil hybrid 24 and in series with the lower pair of linewindings of the repeating coil hybrid.

The composite of signaling currents are transmitted over lines 22V and23 to the intermediate repeater point where they are impressed 'oncomposite signaling equipment |63. This equipment may incorporate meansfor repeating the received composite signals into lines 45 and 46.

Retardation coils |65, |66, |61 and |1| serve to exclude voice frequencycurrents from the path provided for the composite signals at theintermediate repeater point.

At the east terminal composite signals received over lines 45 and 46,after passing through the respective line windings of repeating coilhybrid 41 and respective composite set retardation coils |12 and |13,are impressed on the receiving apparatus of the east terminal compositeset associated with side circuit No. 1. Any voice frequency current thatmay happen to find its way into the direct current path will be drainedo through the composite signaling or dialing equipment or to ground |14through capacitances |15 and |16. Ground |11 and capacitances |18 and 19and similar grounds and capacitances at the intermediate sections servealso to drain off noise currents from the composite signaling or dialingequipment which otherwise might enter the line circuits of the repeatingcoil hybrids.

It will be understood (that transmission of composite signaling currentover side circuit'No. 2 is accomplished in a manner similar to thatdescribed above and it should be particularly noted that the novelarrangement of the repeatcoils of the intermediate section of sidecircuit No. 2 (where circuits are on a non-repeatered basis) is suchthat the line equipment of side circuit No. 1 and side circuit No. 2 atthe intermediate point is the same; this is a particularly valuablefeature not only in view of the resulting advantages when the circuitslare as disclosed but also in view of the fact that should theintermediate section circuits of side circuit No. 2 subsequently beplaced on a repeatered basis, such change can be introduced withoutnecessitating any change in the composite signaling path.

To briefly sum up some of the outstanding advantages of the novelarrangement contemplated by the present invention and disclosed asapplied to the intermediate section of side circuit No. 2;

the first advantage to be noted is that while the electricalcharacteristics of the modified arrangement are such that the equivalentof a single coil having an impedance ratio slightly higher than that ofa normal repeating coil hybrid is produced whereby good performance bothfrom a transmission eiciency and a return loss standpoint is achieved,the physical structure of the modified device is such that conversion toa normal repeating coil hybrid involves only a few simple rearrangementsof the electrical strappings. An additional advantage of the modiedarrangement is that the drop side of the coil is well balanced to groundthereby facilitating mid-point composite signaling. Among otheradvantages is the fact that despite the necessary unsymmetricalarrangement resulting when the circuits of one of the side circuits arenon-repeatered at a particular section, the method of deriving thephantom circuit and the method of transmitting direct current and lowfrequency alternating current signals at the unsymmetrical points areexactly the same as the method followed at normal hybrid points.Obviously, this is a particularly advantageous feature as it greatlyfacilitates changing the circuits of a section from a non-repeatered toa repeatered basis or vice versa.

out of the system and the in hybrid and out hybrid windings of therepeating coil hybrid are connected in parallel aiding with each otherand directly to the drop leads of the local switchboard circuits. Thisresults in a much lower transmission loss than is encountered in similaremergency systems of the prior art wherein the repeater elements aloneare shunted out of the line upon their failure.

Terminal line section |91, under normal operating conditions, isconnected through the make contacts of relay |92 to terminal resistancehybrid |93'. Under similar normal operating conditions the `output pathof west-east amplifier |34 is connected through make contacts of relay|35 Yto in hybrid windings |96 of repeating coil hybrid 191 and theinput path of eastwest amplier 26| is connected through make contacts ofrelay V202 to out hybrid windings 203 of the repeating coil hybrid.

As schematically indicated the energizing current for the laments of theamplifying elements is supplied from power pack 214 over lines 2|5 whilethe plate potential for amplifier 20| is supplied over line 206, throughthe operating winding of relay 201 and over line 221 while the platepotential for amplifier |34 is supplied over line 206, through theoperating winding of relay 222 and over line '223.

Under normal conditions, i. e., when the proper plate potential is beingsupplied by power pack 2|4 to the amplifiers (relays 201 and 222 areheld in operated position due to the passage of the plate currentthrough the operating winding thereof. So long as relays '201 and222`remain in operated position relays |35, 202 and |92 are likewiseheld operated over a path from the plate potential output of power pack2M, Vline 221|, operating windings of relays |35 and '202, line 225,operating Awinding of relay |92, make contact of relay '201 to ground226 at the make contact of relay 222.

So long as power pack V211i is functioning properly therefore and thenecessary plate potential is being supplied, circuit conditions will beas illustrated and voice frequency currents originating in line section|91 will be amplified by West-east amplier |34, applied to in hybridwindings |96 of 'repeating coil hybrid '|91 and appear finally in linesection 221 for out transmission. Under similar conditions, voicefrequency currents coming in over line section 22,1 will be applied toout hybrid windings 203 of the repeating coil hybrid, amplied byeast-West amplifier 20| and applied to line section |9|.

Let us assume now, however, that the flow of plate potential fails, forexample, in the supply path of amplier 20|. Relay 201 will restore tounoperated position due to the cessation of the flow of currenttherethrough thereby interrupting at its make contact the path overwhich relays 135, 202 and |92 had been operated; the threelast-mentioned relays restore to unoperated position.

Relay |92 now being in unoperated position, terminal line section |9l isdisconnected from terminal resistance hybrid |93 and connected insteadto line 23|. Relays |35 and 252 likewise being in unoperated position,windings |96 and 203 of repeating coil hybrid |91 are disconnected fromrespective amplifiers |34 and 20| and connected, instead, in parallelaiding relationship with each other and directly through line 29| toline section |9I. The resulting structure bears a general similiarity tothe modified repeating coil arrangement described above in thatconnection of lwindings |95 and 203 of the two cores in.

parallel aiding relationship results, in eiiect, in electricallyymerging the two coils and causing them `t0 function as a single coil.

Voice frequency currents originating at this time in line section |9|will pass directly to windings |96 and `203 of repeating coil hybrid |91over line 23| and will be impressed through inductive action on linesection 221. Similarly, voice frequency currents coming in over linesection 221 are repeated through inductive action in windings ISS-and203 and passed therefrom directly to line section l9| over line 23|.This novel switching arrangement whereby, upon repeater failure, theterminal resistance yhybrid and the ampliers are shunted out of `thecircuit and the in hybrid and out hybrid windings of the .repeating coilhybrid connected in parallel yaid-ing relationship with each yother and.directly to the terminal line section has been found to be effective inappreciably reducing the transmission loss resulting from repeaterfailure. The switching operations, however, do not interfere with theoperation of ythe over-al1 system particularly with respect to phantomderivation and direct lcurrent and low frequency alternating currentsignaling.

It will be clear, of course, that the switching operations are initiatedin a similar manner upon failure of the ilow of plate .potential in thesupply path of amplifier |311.; in the latter event the path over whichrelays |135, 202 and |92 are held operated would be interrupted at themake contact .of relay 222. It will .be clear, further, that the.arrangement is applicable to direct current energized ampliers as wellas .altern-ating current .energized amplifiers.

While .the `emergency .switching .arrangement has been disclosed asapplied to a terminal section :it will be understood that .thearrangement may be ,applied as well to an intermediate repeater section,

While certain specic embodiments of the invention .have been .selected4for .detailed .disclosure the .invention Iis not, of course, limited inits application to the embodiments disclosed. For `example it is.contemplated that a speech frequency system of the type disclosed may,and in most vcases will, include ymore than one intermediate repeatersection; .the .modified .repeating coil arrangement may of course be`used at any point Where circuits are on Va non-repeatered basis. Alsowhile the .modified repeating coil arrangement .has beenshown applied toone .side circuit of a phantom .group it will .be .understood that itmay be applied to .more than one oi the circuits of a phantom group.Further itis contemplated that coils Ybe used of the `type havingmultisection windings and that various polings and interconnections ofthe sections be followed in order to obtain impedance ratios requiredbyparticular situations. (The 4equations for -figuring the impedanceratios 'for -t-he type' of coil,

with which the invention is concerned are given above.) In short, theembodiments disclosed should be taken as illustrative of the inventionrather than as restrictive thereof.

What is claimed is:

1. In a speech transmission system, a first side circuit including aterminal section and an intermediate repeater section, a rst two-wireline connecting said sections, a second side circuit including aterminal section and an intermediate non-repeatered section, a secondtwo-wire line connecting said two last-mentioned sections, a two-Wayrepeater for voice frequency current at said intermediate repeatersection, a repeating coil hybrid connecting said rst two-wire line totwo sides of said repeater in mutually conjugate relationship, means forderiving a phantom circuit from said two side circuits, an intermediatephantom repeater section, means at said intermediate sections of saidtwo side circuits for picking ofiF phantom currents for application tosaid intermediate phantom repeater section, a repeating coil setconnecting said second twowire line to equipment of said intermediatenonrepeatered section, said repeating coil set comprising a pair ofmagnetic cores, a first primary winding and a second primary winding onthe first of said cores, a rst primary winding and a second primarywinding on the second of said cores, said rst primary windings beingconnected in series with each other and with one side of said secondtwo-wire line, said second primary windings being connected in serieswith each other and With the other side of said second two-wire line, apair of secondary windings on each of said cores, means for connectingsaid pairs of secondary windings in parallel aiding relationship witheach other, and an external circuit connected to said secondarywindings.

2. In a speech transmission system, a first side circuit including aterminal section and an intermediate repeater section, a first two-wireline connecting said sections, a second side circuit including aterminal section and an intermediate non-repeatered section, a secondtwo-wire line connecting said two last-mentioned sections, said rst andsecond two-Wire lines being balanced to ground, a two-way repeater forvoice frequency current at said intermediate repeater section, arepeating coil hybrid connecting said rst twowire line to two sides ofsaid repeater in mutually conjugate relationship, means for deriving aphantom circuit from said tWo side circuits, an intermediate phantomrepeater section, means at said intermediate sections of said two sidecircuits for picking off phantom currents for application to saidintermediate phantom repeater section, a repeating coil set connectingsaid second two-wire line to equipment of said intermediatenon-repeatered section, said repeating coil set comprising a pair ofmagnetic cores, a rst primary winding and a second primary winding onthe rst of said cores, a first primary winding and a second primarywinding on the second of said cores, said rst primary windings beingconnected in series with each other and with one side of said secondtwo-wire line, said second primary windings being connected in serieswith each other and with the other side of said second two-wire line, apair of secondary windings on each of said cores, means for sointerconnecting said pairs of secondary windings that said repeatingcoil set is balanced to ground on the secondary winding side, and anexternal circuit connected to said Secondary windings.

Y 3. In a speech transmission system of the type including two sidecircuits and a phantom circuit derived from said side circuits, aphantom group at a point in the system including a repeatered sidecircuit section, a phantom section and a non-repeatered side circuitsection, a twoway voice frequency repeater for said repeatered sidecircuit section, a repeating coil hybrid connecting the lines of one ofsaid side circuits to two sides of said repeater in mutually conjugaterelationship, said repeating coil hybrid comprising a pair of magneticcores and a plurality of windings thereon, voice frequency"terminatingequipment associated with said non-repeatered side circuit section, apair of repeating coils connecting the lines of the other of said sidecircuits to said voice frequency terminating equipment, each of saidrepeating coils comprising a magnetic core provided with a plurality ofwindings thereon and means for so interconnecting certain of saidwindings of said repeating coils that the two coils function as theelectrical equivalent of a single coil having an impedance ratiosubstantially equal to but slightly greater than the impedance ratio ofsaid repeating coil hybrid.

4. In a speech transmission system of the type including two sidecircuits and a phantom circuit derived from said side circuits, thelines of said side circuits being balanced to ground, a phantom group ata point in the system including a repeatered side circuit section, aphantom section and a non-repeatered side circuit section, a twowayvoice frequency repeater for said repeatered side circuit section, arepeating coil hybrid connecting the lines of one of said side circuitsto two sides or" said repeater in mutually conjugate relationship, saidrepeating coil hybrid comprising a pair of magnetic cores and threepairs of inductive windings on each of said cores, said windings beingpositioned on the respective cores and interconnected in a predeterminedmanner, voice frequency terminating equipment associated with saidnon-repeatered side circuit section, a pair of repeating coilsconnecting the lines of the other of said side circuits to said voicefrequency terminating equipment, each of said repeating coils comprisinga magnetic core provided with three pairs of windings thereon, saidrepeating coils being with respect to the number of windings andarrangement thereof on the respective cores the exact physical duplicateof said repeating coil hybrid, means for connecting certain windings ofeach of said repeating coils in series aiding relationship with eachother and to the lines of said other side circuit, and means forconnecting other windings of each of said repeating coils in parallelaiding relationship with each other and to said voice frequencyterminating equipment.

5. In a speech transmission system, a rst line section and a second linesection, a two-way repeater for Voice frequency currents, a resistancehybrid for connecting said first line section to two sides of saidrepeater in mutually conjugate relationship, a repeating coil hybrid forconnecting said second line section to two sides of said repeater inmutually conjugate relationship, said repeating coil comprising twocores, a plurality of windings on each of said cores, a pair of saidwindings on one of said lcores serving under normal operating conditionsof said repeater to inductively couple said second line section to saidi'lrst ,line section over a path including one directional path of saidtwo-way repeater and said resistance hybrid, a pair of said windings onthe mally to connect said two pairs of said windings in parallel aidingrelationship with each other and directly to said first line sectionover a path excluding both directional paths of said re- 5 peater andsaid resistance hybrid.

MY RON R. KLEIST.

